A New ONO3- Trianionic Pincer-Type Ligand for Generating Highly Nucleophilic Metal–Carbon Multiple Bonds
2012
O’Reilly, Matthew E. | Ghiviriga, Ion | Abboud, Khalil A. | Veige, Adam S.
Appending an amine to a C═C double bond drastically increases the nucleophilicity of the β-carbon atom of the alkene to form an enamine. In this report, we present the synthesis and characterization of a novel CF₃–ONO³⁻ trianionic pincer-type ligand, rationally designed to mimic enamines within a metal coordination sphere. Presented is a synthetic strategy to create enhanced nucleophilic tungsten–alkylidene and −alkylidyne complexes. Specifically, we present the synthesis and characterization of the new CF₃–ONO³⁻ trianionic pincer tungsten–alkylidene [CF₃–ONO]W═CH(Et)(OᵗBu) (2) and −alkylidyne {MePPh₃}{[CF₃–ONO]W≡C(Et)(OᵗBu)} (3) complexes. Characterization involves a combination of multinuclear NMR spectroscopy, combustion analysis, DFT computations, and single crystal X-ray analysis for complexes 2 and 3. Exhibiting unique nucleophilic reactivity, 3 reacts with MeOTf to yield [CF₃–ONO]W═C(Me)(Et)(OᵗBu) (4), but the bulkier Me₃SiOTf silylates the tert-butoxide, which subsequently undergoes isobutylene expulsion to form [CF₃–ONO]W═CH(Et)(OSiMe₃) (5). A DFT calculation performed on a model complex of 3, namely, [CF₃–ONO]W≡C(Et)(OᵗBu) (3′), reveals the amide participates in an enamine-type bonding combination. For complex 2, the Lewis acids MeOTf, Me₃SiOTf, and B(C₆F₅)₃ catalyze isobutylene expulsion to yield the tungsten–oxo complex [CF₃–ONO]W(O)(ⁿPr) (6).
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